The Terminal app provides you a console or terminal emulator window to enable access to the unix sub system.
Basically most generic unix command will work at this level.
Most basic unix tutorials will cover the commands you can use here and Apple has man (Manual) pages on most of the commands. Just type
$ man what_ever_command_you_want_to_know_about &lt;Return&gt;

Some of the more generic commands include
ls to do see the directory
cd to change directory
cat to view a file
ps to see process details
df to see disk usage
rm to remove files
exit to logout

You probably won't need to use the terminal app unless you want to get into the more nitty gritty of the OS.

Brief tutorials will get you into trouble in unix they are a unix newbies worst nightmare. They tell you a few commands - some of which can do you more harm than good., Also, they don't give you a true understanding of what your dealing with.
The Sams book I suggested is a fine place to start your quest for unix knowlege